<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="content-type" content="text/html;charset=UTF-8">
    <title>center</title>
</head>
<body>
    <!--border边框，solid 实线，margin 内边距。-->
    <div style="width:500px;height:300px;border:1px solid #ddd;margin:0 auto"></div>
</body>
</html>